There are many applications in which it is required to mix two powders or to prepare a mix of a powder and a liquid. In particular in the building trade, one may need to mix flowable solids such as sand, cement and ballast and to add water to form wet concrete.
Hitherto, the conventional method used for mixing powders with one another and with liquids has been to churn the ingredients. Cement mixers used for preparing plaster, mortar, cement and concrete use a drum that constantly rotates in the same direction and has an internal screw for turning the constituents over or churning them until they are thoroughly mixed.